A Canada goose is a Canada goose is a Canada goose, right? Wrong – not anymore.

With the 45th supplement of its official checklist published in July 2004, the American Ornithologist’s Union (AOU) split the large and varied species Branta canadensis into two species: Branta Canadensis, the Canada goose, and Branta hutchinsii, the cackling goose.

Many ornithologists and birders have long argued for some sort of split among the large and varied group of Canada geese, which before the split contained North America’s largest goose, the giant Canada goose as well as one of North America’s smallest — the cackling goose.

According to field guide author David Allen Sibley, the AOU has been studying the Canada complex for a number of years with some suggesting as many as four separate species. It appears the split into two groups was based primarily on DNA studies.

The DNA showed that the entire Canada complex originated from a common ancestor. However, it also showed considerable difference between today’s smaller, more northerly nesters and their larger more southerly cousins. Researchers believe that nesting populations of Canadas became fragmented during the last Ice Age and that the two groups have evolved separately since that time.

The two species break down this way: Branta canadensis (the large-bodied group) includes seven subspecies; Branta c. canadensis, the Atlantic Canada goose; Branta c. interior, the Hudson Bay Canada goose; Branta c. maxima, the giant Canada goose; Branta c. moffitti, the Moffitt’s or great basin Canada goose; Branta c. parvipes, the lesser Canada goose; Branta c. occidentalis, the dusky Canada goose and Branta c. fulva, the Vancouver’s Canada goose.

Branta hutchinsii, the cackling goose, includes four subspecies; Branta h. hutchinsii, the Richardson’s cackling goose; Branta h. taverneri, the Taverner’s cackling goose; Branta h. minima, the cackling cackling goose and Branta h. leucopareia, the Aleutian cackling goose.

For the most part it is easy to see the body and bill size differences between the Canada goose and the cackling goose, and it’s easy to hear the difference in voice (cackling has a higher-pitched call) making the split look like a simple matter. But nature aficionados know nature is never that tidy.

Between the extremes of large-bodied Canadas and small-bodied cackling geese are a couple of intermediate species of relatively the same size and bill dimensions. They are Branta c. parvipes the lesser Canada goose and Branta h. taverneri the Taverner’s cackling goose.

The lesser Canada is said to be lighter overall with a paler breast but birders, ornithologists and biologists say there are many intermediate individuals. In fact, in The Birds of North America Mowbray et al listed all the Canadas on the North Slope of Alaska as taverneri/parvipes. And while field studies suggested widespread interbreeding, DNA testing showed little evidence of hybridization and placed the lesser squarely in the Canada group and Taverner’s with the cackling goose. Scientists say the similarity between the two species is not caused by hybridization but rather by a process known as evolutionary convergence – two species evolving towards each other.

Birders in the field confronted with distinguishing between Taverner’s and lesser will have to rely on a combination of factors and may still be left with a guess. The most notable differences are probably: Taverner’s darker overall with darker breast, chunkier more compact body with square-looking head and bill; lesser is lighter with lighter breast, more slender body and rounded head.

Most birders in the East won’t be faced with that dilemma. The most common cackling goose in the East is likely Branta h. hutchinsii the Richardson’s. However, eight cackling geese believed to be Branta h. minima, the cackling cackling goose were reported from Mattamuskeet during this year’s Christmas Bird Count.

The Richardson’s nests in Arctic Canada and winters in south Texas and Mexico.

Richardsons resemble miniature Canada geese. They are closer in size to a mallard, however. Their backs are more silvery or frosted than their much larger cousins and their heads are square and steep with short, stubby bills.

Branta h. minima is primarily a western species nesting on the northwest coast of Alaska and wintering in southern California. They are even a bit smaller than Richardson’s and darker overall with a dark breast, rounded head and short stubby bill.

The cackling goose was apparently proud of its new species designation and showed up on a couple of east Tennessee Christmas counts as well as the Balsam count and the Mattamuskeet count in North Carolina. One individual was also recorded on Lake Julian in Asheville.

The new nomenclature is a bit quirky. Cackling goose is the common name given to the new species. In the past it referred to the smallest of the subspecies Branta c. minima. But, following scientific protocol, the new species name Branta hutchinsii is derived from the first named subspecies. This is how we are left with the redundant cackling cackling goose for Branta h. minima.

(Don Hendershot can be reached at ddihen@juno.com)